Bumper beams or cross members for vehicles, particularly automobiles, are made from high strength or ultra-high strength steel. It has been difficult to manufacture the bumper beam or bar out of some high strength steel because such bar requires a curvature to be placed therein. It is necessary to roll form the cross member to obtain the desired curvature. In the past, when high strength steel is roll formed into the desired sweep or curvature, the material of the cross members has been damaged during the process resulting in unacceptable quality. The high strength obtainable with such steels is desired in order to provide a bumper beam or bar which can be impacted at speeds of up to 5 mph without resulting damage.
It is desired for certain applications where high strength is needed to use higher carbon steels, frequently alloyed with other materials. Such steels are characterized as “high-strength steels”. Within this category, there is a class defined as “ultra-high strength steel”. Such steel has a minimum yield of 100 ksi (100,000 pounds per square inch). Chemical compositions for such steel vary from one producer to another. Different compositions and thermo-mechanical processing may produce equivalent results. Formability of high strength steel is difficult than with low carbon steels because of greater spring-back and reduced ductility.
Thus, industry requires that bumpers maintain a high level of strength and damage resistance to meet the expectations of the consumer and government regulations throughout the world concerning low speed vehicular impact. The bumper must be of low weight to minimize vehicle dead weight which reduces gas mileage and increases suspension requirements. Also, bumpers must have a low manufacturing cost and a high dimensional consistency. Thus, high strength-to-weight ratios and ease of manufacture are of importance to the automotive industry.
Various systems, methods and equipment have been employed to manufacture tubular roll-formed automotive bumpers or cross members such as disclosed in the Sturrus et al U.S. Pat. Nos. 5,306,058, 5,092,512 and 5,104,026, each relating to a tubular bumper beam, method and/or apparatus for roll forming an automotive bumper. The same equipment and methods employed in the foregoing patents have also been used to manufacture “B” section cross members as disclosed in the Sturrus' U.S. Pat. No. 5,395,036 entitled “Method of Roll Forming an End Automotive Bumper”; and U.S. Pat. Nos. 5,454,504, 5,566,874 and 5,813,594, entitled “Apparatus For Forming an End Bumper for Vehicles”.
A disadvantage of the methods and apparatuses disclosed in the Sturrus and Sturrus et al patents is that the method and apparatus for forming the straight tubular “B” section cross member and the equipment for forming the sweep in the cross members are connected together in one line, with the severing of the cross member from the roll formed member being located down stream of the welding station. With such a process, the curvature or sweep is placed in the cross member prior to cutting the swept integral tube.